An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles dispersed in a solvent or solvent mixture. An EPD typically comprises a pair of spaced-apart electrode layers. At least one of the electrode layers, typically on the viewing side, is transparent. An electrophoretic fluid composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode layers.
An electrophoretic fluid may have one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is imposed between the two electrode layers, the charged pigment particles migrate by attraction to the side of polarity opposite that of the pigment particles. Thus, the color showing at the transparent electrode layer side may be either the color of the solvent or the color of the pigment particles. Reversal of polarity of applied voltage difference will cause the particles to migrate to the opposite side, thereby reversing the color.
Alternatively, an electrophoretic fluid may have two types of pigment particles of contrasting colors and carrying opposite charge polarities, and the two types of pigment particles are dispersed in a clear solvent or solvent mixture. In this case, when a voltage difference is imposed between the two electrode layers, the two types of pigment particles would move to the opposite ends. Thus one of the colors of the two types of the pigment particles would be seen at the viewing side.
In another alternative, multiple types of pigment particles of different colors are present in an electrophoretic fluid for forming a highlight or multicolor display device.
U.S. Pat. Nos. 6,930,818 and 6,933,098 describe technology for the preparation of an electrophoretic display based on cup-like microcells. The contents of both patents are incorporated herein by reference in their entirety.
Briefly, the microcells may be prepared by microembossing or imagewise exposure, and they are filled with an electrophoretic fluid. The filled microcells are then sealed with a sealing layer, which can be accomplished by a one-pass method or a two-pass method. The electrophoretic display film may also comprise an adhesive layer and a primer layer.
The microcell layer, sealing layer, adhesive layer and primer layer in an electrophoretic display preferably have an electrical resistivity within a certain range, 107 to 1010 ohm·cm, in order to ensure adequate optical performance of an electrophoretic display. This desired level of electrical resistivity, however, is lower than the electrical resistivity of the polymeric materials available for forming these layers.
In order to achieve the desired electrical resistivity, dopant molecules may be added into a composition for forming the dielectric layer, to reduce the electrical resistivity. However because dopant molecules are very active molecules, there is a risk that they may travel into other components of a display device. For example, in the case of a sealing layer which is in contact with an electrophoretic fluid, introduction of the dopant molecules into fluid, even in a minute amount, may cause the performance of the display device to be negatively impacted.
Alternatively, humectants may be added into a composition for forming the dielectric layer to raise the moisture content in the composition, resulting in a lower electrical resistivity. Such approach, however, would render the display device to be extremely moisture dependent and its performance could become sensitive to environmental temperature and moisture changes.